KVM: arm64: Enforce absence of FEAT_FGT2 on FGT2 registers

KVM: arm64: Enforce absence of FEAT_FGT2 on FGT2 registers

Similarly to the FEAT_FGT registers, add the dependency between
the registers and the controlling feature.

WHile we're at it, add the missing checks for the RES0 vs valid
bit overlap.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>